1. Field of the Invention
The present invention relates to a shadow mask and a cathode ray tube, especially a color cathode ray tube, i.e., a color-CRT.
2. Description of the Related Art
Generally, a color-CRT reproduces a color image by selectively passing electron beams emitted from an electron gun through a shadow mask in which a large number of electron beam apertures are formed, and causing the electron beams to bombard predetermined positions of a phosphor layer formed on the inner surface of a glass envelope. In this case, about 1/3 or less of all of the emitted electron beams passes through the electron beam apertures, and the rest of the electron beams directly bombards the shadow mask to heat it. If the shadow mask is thus heated to cause thermal expansion, the positions of the electron beam apertures are displaced from the design criterion, leading to a color misregistration on the phosphor screen.
For this reason, in place of a conventional steel material consisting of low-carbon rimmed cold-rolled steel or low-carbon aluminum-killed cold-rolled steel, an Fe--Ni-based alloy having a small thermal expansion coefficient, such as a 36-wt % Ni--Fe alloy (invar alloy), has become widely used as a shadow mask material recently.
This Fe--Ni-based alloy has the advantage of an extremely small thermal expansion coefficient but also has the disadvantage of a low hardness. As an example, in a process of manufacturing a shadow mask plate material from the invar alloy, if the shadow mask plate material is softened through annealing, the vickers hardness of the resultant plate material becomes approximately 120 to 140. Consequently, a shadow mask made from this plate material resonates with external vibrations, e.g., vibrations from loudspeakers. In the case where shadow mask resonates, a positional difference is produced between a large number of electron beam apertures formed in the shadow mask and a phosphor layer, resulting in a color misregistration. Especially in recent color-CRTs, the diameter and the pitch of the electron beam apertures tend to be decreased in order to realize a high definition and a high image quality. If the positional difference is caused between the electron beam apertures and the phosphor layer, therefore, the degree of the color misregistration further increases.
On the other hand, requirements for the characteristics of color-CRTs, particularly color-CRTs for industrial purposes, such as displays, have become stricter every year, and so a demand has arisen for images with a higher definition and a higher quality. In view of these strict requirements for the characteristics of color-CRTs, therefore, preventing a color misregistration by suppressing howling of the shadow mask becomes important in achieving a higher definition and a higher quality of images.